1. Field of the Invention
This invention pertains generally to corrosion prevention on the surfaces of hollow metal structures, more specifically the placement of a plastic polymer foam material in the box section frame rails of a vehicle to prevent corrosion from salt water collection or condensation in the frame rails.
2. Description of the Related Art
Corrosion control methods are generally based on the principle of shielding metal surfaces from their environment through the application of fluid or vapor impervious surface layers or coatings. These layers can be formed from a variety of materials including oils and greases, paints, inhibiting surface films, inactive metal overlays, or thickened oxide layers. Coating reliability depends on adhesion strength and resistance to ultraviolet light, heat, mechanical, or moisture induced degradation.
Hollow cavities are an intrinsic design feature of metallic structures. For vehicles located near salt water, particularly these cavities tend to collect salt laden moisture which in turn promotes accelerated corrosion of the internal surfaces. Corrosion damage inside structural voids results in premature component failures, significantly shortened lifetimes, and high repair or replacement costs. Internal cavity corrosion is commonly observed in automobiles and trucks causing some vehicles to be removed from service in less than one quarter of their design lifetimes and resulting in high repair costs. For example, the beach vehicles utilized by the Los Angeles County Parks and Harbors Department are retired from service after approximately two years even though they are rinsed with salt-free water daily.
The use of corrosion control treatments such as, paints, oils, or inhibiting coatings, for hollow components is often impracticable because of the fact that there is inadequate accessibility for application. Inaccessibility also reduces the effectiveness of cleaning and surface preparations necessary for corrosion product removal prior to the application of coatings. Application of coatings prior to assembly is not always feasible because of possible deleterious effects of welding on the coating. Providing drainage is only effective if all moisture and residue can be eliminated or the interior surfaces have a good corrosion preventive coating.
Plastic foams are widely used for building insulation and recreational boat hulls, as a material for flotation devices and as space fillers to avoid maintenance in steel sail boat construction. Although corrosion control is not the primary purpose for foam application in these applications, it has been noted that virtually no corrosion appears on foam encased steel spade rudders, even after prolonged periods of ocean sailing. In the early 1960s foam fillers were explored as a corrosion control method for automotive body applications. This concept was subsequently abandoned because of void and shrinkage problems associated with the foam formulations then available. It has been shown that stainless steel, galvanized iron, and copper appear to be protected by contact with some rigid insulating polyurethane foams, however for mild steel and aluminum the protection afforded is significantly less. It has been suggested that in the latter cases, detrimental species will leach from the foam under conditions of high humidity and condensation and support corrosion attack at the foam metal interface.